HISTORY OF ZOOLOGY. 65 



pearance, and preserve this also through the following gen- 

 erations, so long as they remain in the rich soil ; but the 

 plant quickly returns to its former appearance when re- 

 placed in its previous conditions of existence. 



In general, a change seems to be the more permanent 

 the more slowly it has developed. In researches upon the 

 influence of the conditions of existence, we can, therefore, 

 rely soonest upon results, if we experiment with slowly- 

 working factors, such as light and heat, dry or moist air, 

 different intensities of gravitation, of stimuli, etc., which 

 can be excluded from the environment of the organism. 



Use and Disuse. Now, in regard to the efficiency of 

 use and disuse, there is no doubt that the shape of an animal 

 is influenced in a high degree by the manner in which the 

 organs are used. The organs which are much used will be- 

 come especially strong in him, and i'icc rcrsa those which 

 are debarred from use will become weak. It is only ques- 

 tionable whether these, in the strongest sense of the word 

 newly-acquired characteristics thus arising, are transmitted to 

 the offspring, or whether the descendants, in order to attain 

 to the same degree, must not begin anew in the same way use 

 and disuse. In the latter case the cumulation of characteris- 

 tics, and therefore the possibility that these may become per- 

 manent, is excluded. It is to be regretted that accurate 

 results are still lacking on a point so well adapted for 

 experimental treatment. At this time vestigeal organs 

 speak most strongly in favor of the Lamarckian principle ; 

 for we see that cave animals, which for many generations 

 have lived in darkness, are blind, either having no longer 

 any eyes at all, or only vestiges of such, quite incapable of 

 function. This seems to justify the view that this condi- 

 tion is attributable to lack of use, since it has brought 

 about a functional and anatomical incapacity, which has 

 increased from generation to generation. Now we must 

 believe that what is true for disuse must express itself in 

 the reverse sense in the case of use. 



Nageli's Principle of Progression. In conclusion, 

 there is still to be considered the change of species from sub- 



